A widely used method of measuring blood velocity uses the Doppler shift in ultrasound backscattered by red blood cells in moving blood. Ultrasound frequencies that can penetrate the body, for example frequencies in the range of 1-10 Megahertz (MHz), are Doppler-shifted by frequencies that fall in the human range of hearing when they strike blood velocities found in the body, such velocities ranging from about 10 centimeters/second to 5 meters/second. Therefore, when backscattered ultrasound is coherently detected with the transmitted frequency, the Doppler shifted signal, which is proportional to the blood velocity, is audible. This led to the early use of Doppler in medical diagnoses.
With the development of spectrum analysis to quantitate the Doppler shifts, the importance of audible Doppler diminished, as the blood velocity can be found directly from the spectrum.
The audible Doppler signal is still used to find blood vessels and to check for their patency. In addition, there exists a new class of screening ultrasound systems, for example “An Instrument for Screening for Carotid Disease”, in the “Proceedings of the 2005 IEEE International Ultrasound Symposium” (incorporated herein by reference) that utilizes Doppler-audible signals to guide an inexperienced operator in moving the ultrasound probe over the carotid arteries. To find the diseased portions of the carotid system, the operator must detect what can be a faint, high-frequency whisper, indicating the high velocity caused by a stenosis, among other, louder signals produced by normal portions of the carotid arteries.
While previous Doppler instruments have used filters to help in that regard, it has been a challenging situation for inexperienced operators; for screening to succeed, many inexperienced operators will be required. Interpreting the audible Doppler signal is an obstacle to such screening systems.
Alternative methods and apparatus that facilitate detection of Doppler audio signals from diseased vessels, particularly by inexperienced operators, is desired.